1. Field of the Invention
The present invention pertains to optical fiber sensors for detection and localization of perturbation acting upon a structure. More specifically, this invention has reference to distributed optical fiber sensors where an external perturbation such as temperature, pressure, displacement, etc introduces additional losses in the sensing fiber. Particularly, this invention relates to an optical fiber sensor suitable to be used as a leakage detector in an industrial system for the chemical and petrochemical industries that provides measurement of the value and position of the perturbation.
2. Description of the Related Art
Sensors that can detect the presence or absence of a selected substance (xe2x80x9canalytexe2x80x9d) are widely used in many different fields. For example, environmental sensors are used to detect pollutants and have become increasingly important, as environmental standards have become stricter. Medicine is another field where the use of sensors is growing, especially in the area of medical diagnostics. Further, sensors also control various industrial processes.
The prior art sensors for the detection of analytes generally rely on small changes in the indices of refraction in response to the presence of an analyte. Commonly used optical sensors include planar waveguides, optical fibers and diffraction gratings. Optical fiber sensors for sensing the presence of a particular fluid substance such as water vapor, water, petrochemicals, hydrocarbon fuels, etc., are well known in the prior art.
The prior art includes many attempts to provide reliable detection of leaks of the contents of tanks, pipelines, and the like. Detection of leaks in a simple, efficient and reliable fashion is highly desired at present because of increased public awareness of the sensitivity of the environment to chemical spills and the like. Moreover, increasing regulatory activity mandates reduction of industrial leakage of toxic chemicals and the like, and detection and cure of such leaks before their effects can become dangerous or catastrophic.
Hydrocarbon leak detecting devices often have been constructed in a manner, which requires their replacement, or repair upon detection of a leak, that is, once contacted by a hydrocarbon fuel, the detection device, or its key components, must be replaced before the detector can be used again. Another problem is that in most storage tanks or pipelines, there will be considerable ground water present, and any detector must be capable of distinguishing between ground water and hydrocarbons and capable of functioning without being overwhelmed by ground water in order to avoid false detection signals.
U.S. Pat. No. 5,378,889 to Lawrence entitled xe2x80x9cMethod and Apparatus for Detecting Hydrocarbon Fuels in a Vapor State with an Absorber-Expander Memberxe2x80x9d discloses a fiber optic sensor for the detection of hydrocarbon fuels. This type of prior art fiber optic sensor, however, is not optimal. The fiber optic sensor is inaccurate and limited in the distance over which it may be used.
U.S. Pat. No. 4,590,462 to Moorehead entitled xe2x80x9cOil leakage detection apparatusxe2x80x9d employs microbending of an optical fiber in a detection unit to detect hydrocarbon fuels. A rotary actuator is mechanically coupled to an optical fiber to produce microbending of the fiber. The rotary actuator includes a spring mechanism having stored energy, which is released upon degradation of shear pins under the action of hydrocarbons. Thus, when the hydrocarbon analyte is present in sufficient quantity to degrade the shear pins, the spring is released, and the optical fiber displaced to produce a microbend that can be sensed by optical time domain reflectometry. This approach, however, clearly is not reversible since it depends upon destruction of the shear pins upon contact with the hydrocarbon.
The prior art further shows a method for detecting the leak of an analyte by using a fiber optic sensor in conjunction with a substance that swells and mechanically creates a perturbation in, or particularly, a microbending in the optical fiber. The condition can then be readily detected by the attenuation in the signal transmitted by the fiber or by optical time domain reflectometer (OTDR). For example, U.S. Pat. No. 5,138,153 to Gergely et al. entitled xe2x80x9cDistributed Fiber-Optic Sensor with Substance Selective Permeable Coatingxe2x80x9d discloses a fiber optic sensor based upon passing effects in which the cladding has an index of refraction less than the core, and the cladding is sensitized to the analyte. When the analyte contacts the cladding, it increases the index of refraction of the cladding above the core to thereby couple the light transmitted in the core to the evanescent wave. The Gergely et al. patent employs its sensor system in a hydrocarbon tank farm, but the cladding is selected to undergo an increase in the index of refraction. Optical time domain reflectometry is used to locate leaks, and both continuous and pulsed light can be employed to sense liquids and vapors having analytes, which will react with the cladding. The Gergely reference detects the leak in the pipeline, but not the location of the leak.
U.S. Pat. No. 5,015,843 to Seitz et al. entitled xe2x80x9cFiber Optic Chemical Sensors Based on Polymer Swellingxe2x80x9d is directed to a fiber optic system in which polymer swelling is used to mechanically or physically displace a reflective surface coupled to the fiber optic core and thereby influence light transmission back to the detector. The system requires a relatively high concentration of analyte to be effective, and in order to enhance a sensitivity and minimize this disadvantage, the system preferably is miniaturized.
In addition to leak detection of pipelines, the location of the leak, monitoring and measurement of structural loads, and perturbations in the pipeline are also important.
In some environments, it is necessary or desirable to monitor the location and magnitude of selected loads acting upon a physical structure. For example, it is highly desirable to locate and quantify localized perturbation to which an oil or gas pipeline is subjected, primarily as a result of variations in weather and ground elevation, so that remedial measures can be taken prior to breakage of the pipeline. This problem of pipeline perturbation is particularly troublesome when the pipeline travels through expansive regions of wilderness or wasteland, such as the so-called Alaskan pipeline, which extends for hundreds of miles over relatively unstable tundra.
U.S. Pat. No. 4,421,979 to Asawa et al., entitled xe2x80x9cMicrobending Of Optical Fibers For Remote Force Measurementxe2x80x9d discloses a system for remote measurement of structural forces, including a plurality of microbend transducers mounted along the length of the structure for microbending an optical fiber in response to structural forces. The reference detects the leak in the pipeline, but not the location of the leak.
The prior art has not provided sensors for detecting and exactly locating the position of leaks from tanks, including tanks buried in or resting on the ground, and from pipelines and other vessels using unmodulated continuous-wave (CW) light source. All prior art distributed sensors are used for the localization of the perturbation time or frequency modulated light sources. In general, prior art leak detectors have been unduly complicated, and thus, both expensive and prone to erroneous signals.
Accordingly, there is a need in the art for a system and device for detecting and localizing leaks as soon as the leak occurs, which system and device are comparatively easy to manufacture, inexpensible, and reliable.
The present invention overcomes the problems encountered in the prior art by providing a practical and effective optical fiber sensor for detecting the magnitude and location acting along the length of a structure, such as an oil or gas pipeline, wherein a large number of loss-inducing transducers are capable of being monitored simultaneously from a single remote monitoring station.
It is therefore a principal object of the present invention to provide a unique fiber optical sensor, which utilizes non-modulated continuous wave light source for detection and localization of the disturbance along the test fiber.
It is an object of the present invention to provide a fiber optic sensor which is easy to install, requires minimum maintenance, is inexpensive to construct, and is easily adjusted.
It is yet another object of the invention to provide a fiber optic sensor to detect the exact location of a leak based upon the unique relation between transmitted and reflected powers for different locations of the disturbance along the test fiber.
It is yet another object of the invention to provide a fiber optic sensor to detect the location of a leak comprising plurality of reflectors and a plurality of loss induced members positioned matching the reflectors.
In view of the foregoing disadvantages inherent in the known types of leak sensors in the prior art, which use a non-modulated continuous wave light source, the present inventor discovered a unique fiber optic sensor, which makes possible not only the detection, but also the localization, of leaks.
The fiber optic sensor in its basic form comprises:
a test fiber having a first port and a second port;
a light source for producing a beam of light propagating along the test fiber;
a fiber optic beamsplitter having a first port connected to the light source, a second port connected to the first port of the test fiber, and a third and a fourth port;
a plurality of reflectors positioned along the test fiber and a plurality of loss-inducing members positioned along the test fiber, wherein said each of the reflectors is matched to each loss-inducing members, wherein at least one reflector is placed between each consecutive loss-inducing members;
an optical reflection detector to receive a constant light flux, the optical reflection detector connected to the third port of optic beamsplitter, wherein the reflection detector is adapted to sense changes in the power of the light reflected from the reflectors;
an optical transmission detector adapted to receive the light flux, connected to the second port of test fiber, said transmission detector being operable to sense changes in the power of the light transmitted through the test fiber; and
a transmission/reflection analyzer connected to reflection and transmission detectors, said analyzer adapted to measure the value and identify the location of the disturbance along the test fiber by using an unique relation between transmitted and reflected powers for different locations along the test fiber.
In a first preferred embodiment, the plurality of reflectors comprises a set of lumped (group) reflectors such as fiber Bragg gratings or fiber splices.
In a second preferred embodiment, the plurality of reflectors is continuously distributed inside the test fiber.
In a third preferred embodiment, the plurality of loss-inducing members comprises a plurality of bending members.
In a fourth preferred embodiment, the plurality of loss-inducing members comprises a plurality of waveguide-sensitive members, which change the transmission due to change of its waveguide property.
In a fifth preferred embodiment, the plurality of reflectors comprise a set of selective reflectors each preferentially reflective at a non-overlapping different narrow bandwidth of wavelength inside a measurement wavelength range and the light source having broadband radiation range which exceeds or equals the measurement wavelength range.
In a sixth preferred embodiment, the plurality of bending members includes an absorber/expander mechanically coupled to the test fiber to produce a change in transmission of light along the fiber upon absorption of a chemical agent.
In a seventh preferred embodiment, the transmission/reflection analyzer detects the value and identifies the location of the disturbance along the fiber by using the unique relation between transmitted and reflected powers for different locations of the disturbance along the test fiber for both lasers.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood, and so that the present contribution to the art can be more fully appreciated. Additional features of the invention, which will be described hereinafter, will form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other devices and systems for detecting and localizing leakages or for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims.